157 research outputs found
Bare electron dispersion from photoemission experiments
Performing an in-depth analysis of the photoemission spectra along the nodal
direction of the high temperature superconductor Bi-2212 we have developed a
procedure to determine the underlying electronic structure and established a
precise relation of the measured quantities to the real and imaginary parts of
the self-energy of electronic excitations. The self-consistency of the
procedure with respect to the Kramers-Kronig transformation allows us to draw
conclusions on the applicability of the spectral function analysis and on the
existence of well defined quasiparticles along the nodal direction even for the
underdoped Bi-2212 in the pseudogap state.Comment: 4 pages 3 figures revtex, corrected misprint
Change of quasiparticle dispersion in crossing T_c in the underdoped cuprates
One of the most remarkable properties of the high-temperature superconductors
is a pseudogap regime appearing in the underdoped cuprates above the
superconducting transition temperature T_c. The pseudogap continously develops
out of the superconducting gap. In this paper, we demonstrate by means of a
detailed comparison between theory and experiment that the characteristic
change of quasiparticle dispersion in crossing T_c in the underdoped cuprates
can be understood as being due to phase fluctuations of the superconducting
order parameter. In particular, we show that within a phase fluctuation model
the characteristic back-turning BCS bands disappear above T_c whereas the gap
remains open. Furthermore, the pseudogap rather has a U-shape instead of the
characteristic V-shape of a d_{x^2-y^2}-wave pairing symmetry and starts
closing from the nodal k=(pi/2,pi/2) directions, whereas it rather fills in at
the anti-nodal k=(pi,0) regions, yielding further support to the phase
fluctuation scenario.Comment: 6 pages, 4 eps-figure
Surface properties of SmB6 from x-ray photoelectron spectroscopy
We have investigated the properties of cleaved SmB single crystals by
x-ray photoelectron spectroscopy. At low temperatures and freshly cleaved
samples a surface core level shift is observed which vanishes when the
temperature is increased. A Sm valence between 2.5 - 2.6 is derived from the
relative intensities of the Sm and Sm multiplets. The B/Sm
intensity ratio obtained from the core levels is always larger than the
stoichiometric value. Possible reasons for this deviation are discussed. The B
signal shows an unexpected complexity: an anomalous low energy component
appears with increasing temperature and is assigned to the formation of a
suboxide at the surface. While several interesting intrinsic and extrinsic
properties of the SmB surface are elucidated in this manuscript no clear
indication of a trivial mechanism for the prominent surface conductivity is
found
Time-reversal symmetry breaking versus superstructure
One of the mysteries of modern condenced-matter physics is the nature of the
pseudogap state of the superconducting cuprates. Kaminski et al.1 claimed to
have observed signatures of time-reversal symmetry breaking in the pseudogap
regime in underdoped Bi2Sr2CaCu2O8+d (Bi2212). Here we argue that the observed
dichroism is due to the 5x1 superstructure replica of the electronic bands and
therefore cannot be considered as evidence for the spontaneous time-reversal
symmetry breaking in cuprates.Comment: 5 pages, pd
Doping dependence of the mass enhancement in (Pb,Bi)_2 Sr_2 Ca Cu_2 O_8 at the antinodal point in the superconducting and normal state
Angle-resolved photoemission spectroscopy (ARPES) is used to study the mass
renormalization of the charge carriers in the high-T_c superconductor
(Pb,Bi)_2Sr_2CaCu_2O_8 in the vicinity of the (pi,0) point in the
superconducting and the normal state. Using matrix element effects at different
photon energies and due to a high momentum and energy resolution the bonding
and the antibonding bands could be separated in the whole dopant range. A huge
anisotropic coupling to a bosonic collective mode is observed below T_c for
both bands in particular for the underdoped case. Above T_c, the more isotropic
coupling to a continuum or a mode at much higher energy is significantly
weaker.Comment: 4 revtex pages, 4 eps figure
Evolution of Superconductivity in Electron-Doped Cuprates: Magneto-Raman Spectroscopy
The electron-doped cuprates Pr_{2-x}Ce_xCuO_4 and Nd_{2-x}Ce_xCuO_4 have been
studied by electronic Raman spectroscopy across the entire region of the
superconducting (SC) phase diagram. The SC pairing strength is found to be
consistent with a weak-coupling regime except in the under-doped region where
we observe an in-gap collective mode at 4.5 k_{B}T_c while the maximum
amplitude of the SC gap is ~8 k_{B}T_{c}. In the normal state, doped carriers
divide into coherent quasi-particles (QPs) and carriers that remain incoherent.
The coherent QPs mainly reside in the vicinity of (\pi/2, \pi/2) regions of the
Brillouin zone (BZ). We find that only coherent QPs contribute to the
superfluid density in the B_{2g} channel. The persistence of SC coherence peaks
in the B_{2g} channel for all dopings implies that superconductivity is mainly
governed by interactions between the hole-like coherent QPs in the vicinity of
(\pi/2, \pi/2) regions of the BZ. We establish that superconductivity in the
electron-doped cuprates occurs primarily due to pairing and condensation of
hole-like carriers. We have also studied the excitations across the SC gap by
Raman spectroscopy as a function of temperature (T) and magnetic field (H) for
several different cerium dopings (x). Effective upper critical field lines
H*_{c2}(T, x) at which the superfluid stiffness vanishes and
H^{2\Delta}_{c2}(T, x) at which the SC gap amplitude is suppressed by field
have been determined; H^{2\Delta}_{c2}(T, x) is larger than H*_{c2}(T, x) for
all doping concentrations. The difference between the two quantities suggests
the presence of phase fluctuations that increase for x< 0.15. It is found that
the magnetic field suppresses the magnitude of the SC gap linearly at
surprisingly small fields.Comment: 13 pages, 8 figures; submitted to Phys. Rev.
- …